Photodiode apparatus for reducing beam drift of a cathode ray tube display system

ABSTRACT

Apparatus for detecting and correcting drift and position errors of an electron beam due to electrical transients or mechanical vibration in a cathode ray tube display system including a diode array located in the path of the scanning electron beam. When the beam is properly directed, a predetermined output signal matrix from the diode array indicates that the beam is on target; if the beam drifts, a different output signal matrix indicates such drift and a servomechanism coupled to the diode array and to the beam deflection circuitry corrects the deflection of the beam in response to signals from the diode array.

United States Patent [191 Burns July 3,1973

[75] Inventor:

[73] Assignee: Fairchild Camera and Instrument Corporation, Mountain View, Calif.

22 Filed: Jan. 15, 1971 21 Appl. No.: 106,751

Joseph Burns, Pequannock, NJ.

2,571,306 10/1951 Szegho 315/10 Primary Examiner-Carl D. Quanforth Assistant Examiner-J. M. Potenza Aiiorney--Roger S. Borovoy, Alan MacPherson and Charles L. Botsford 5 7] ABSTRACT Apparatus for detecting and correcting drift and position errors of an electron beam due to electrical transients or mechanical vibration in a cathode ray tube display system including a diode array located in the path of the scanning electron beam. When the beam is properly directed, a predetermined output signal matrix from the diode array indicates that the beam is on target; if the beam drifts, a different output signal matrix indicates such drift and a servomechanism coupled to the diode array and to the beam deflection circuitry corrects the deflection of the beam in response to signals from the diode array.

5 Claims, 1 Drawing Figure NORMAL VIEWING AREA ll 2 m monr ARRAY HQ I4 20 15 NORMAL HORIZONTAL SCAN WIDTH W BEAM DEFLECTION 5? AND CORRECTION A 1 CIRCUITRY I I 2s SERVO 24 AMPLIFIER 22 Pnlcnlml July 3, 1973 3,743,883

NORMAL VIEWING [0; ,A/REA f I3 DIODE ARRAY I8 NORMAL HORIZONTAL SCAN WIDTH BEAM DEFLECTION AND CORRECTION GUN Z CIRCUITRY SERVO 25 24 27 AMPLIFIER 22 I NVENTOR.

JOSEPH BURNS ATTORNEY PHOTODIODE APPARATUS FOR REDUCING BEAM DRIFT OF A CATIIODE RAY TUBE DISPLAY SYSTEM BACKGROUND OF THE INVENTION In conventional cathode ray tube display systems, it is often very important that the electron beam remain in a constant scanning pattern free from drift. Registration accuracy of the electron beam is particularly important, for example, in head-up displays for aircraft. These displays provide the pilot with range and azimuth information while he looks at the windshield of the aircraft, avoiding the necessity of taking his eyes off the flight path. There are many other applications of precision cathode ray tube displays which require the type of accuracy provided by the servomechanism of this invention.

BRIEF DESCRIPTION OF THE INVENTION Briefly, the apparatus of this invention for detecting and correction drift of an electron beam in a cathode ray tube display system having conventional beam deflection circuitry includes a plurality of diodes arrayed substantially adjacent the useable face area of a cathode ray tube in an area scanned by the electron beam, such that the array produces a predetermined output signal matrix when the beam is properly directed and is substantially free from drift, but produces a different output signal matrix if the beam has drifted off target. The change in output signal matrices is the result of the beam striking, or failing to strike, one or more diodes which normally would have been struck, respectively, by the beam if the beam were on target; and a servomechanism coupled to the beam deflection circuitry for correcting the deflection of the beam in response to appropriate signals from the diode array indicating that the beam has drifted off target, thereby reducing beam drift and improving the beam deflection accuracy of the system.

The invention in all its ramifications will be better understood from the following more detailed description, making reference to the drawings, in which the single FIGURE of the drawing illustrates, partly schematically, partly pictorially, and partly in block diagram the cathode ray tube display system of the invention having the diode array and servomechanism for improving deflection accuracy.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawings, cathode ray tube 10 has a normal viewing area 11. The border areas 12 of the face of the tube are normally covered by a frame (eliminated from the drawing to allow a better understand ing of the invention) shielding them from the eye of the viewer. The viewer only sees the normal viewing area 11 of the CRT display 10.

In the border area 12 is located a photodiode array 13 which, for purposes of illustration, has four outer photodiodes 14, 15, 16, and 17 and one center photodiode 18. The diode array is powered by a power supply connected to terminal 19 designated V Array 13 is also connected to amplifier 22. The signals then pass through lines 23 to the beam deflection circuitry 24 of the CRT display 25, which includes deflection yoke 27, or to a deflection plate (not shown) within the tube envelope in the case of electrostatic deflection.

The operation of the invention will now be described. During normal operation, electron gun 26 projects a beam of electrons which scans the normal viewing area 11 of CRT 25. The beam is periodically positioned to the reference diode 18 of diode array 13 by reference position voltages, fixed relative to the raster scan and generated by the beam deflection generator. Assuming the beam is perfectly on target, diode array 13 and diodes 14, 15, 16, and 17 are so disposed that the beam, if properly deflected, will completely fail to strike diodes 14, 15, 16, and 17. On the other hand, diode 18, in the center of array 13, will normally be struck by a properly deflected electron beam. The system would also work if diode 18 were deleted and perfect registration produced no output signal.

Accordingly, with the beam deflection being on target, at the time the position on the CRT face of array 13 is scanned, there will be no output signals from diodes 14, 15, 16, and 17, and those of lines 21 connected to those respective diodes will not have a positive output signal. These same signals will also be absent on lines 23, and therefore the beam deflection and correction circuitry 24 will not be put into action. The beam has been and will continue to be deflected in the normal and correct manner across the raster. As a double check, diode 18, which is normally struck by the electron beam when the beam is on target, will provide a positive output signal indicating an on-target beam through one of the lines 21, through servo amplifier 22, and through one of lines 23 to the beam deflection and correction circuitry 24. This output signal will be detected and, in the absence of output signals from any of the other diodes, will provide a normal quiescent signal to the beam deflection and correction circuitry 24. This additional signal merely serves as a double check to be certain of proper beam deflection.

Should the beam drift off target, so as to strike one of diodes 14, 15, 16, or 17, a signal will pass down the associated ones of lines 21, through servo amplifier 22, through the respective ones of lines 23, and into the beam deflection and correction circuitry 24. Depending on which diode, or which combination of diodes, has been struck by the beam, the beam deflection and correction circuitry 24 will be energized and the beam deflection will be corrected to bring the beam back into its proper deflection so as no longer to strike the one or more of these diodes which, during correct beam deflection, should not be struck. The beam deflection is corrected simply by X-Y coordinate D.C. positioning voltages presented to the X and Y yoke windings of the deflection circuits. Scan voltages and related diode reference voltages remain fixed in time and amplitude. Circuitry applicable for so correcting this beam deflection in response to error signals from the photodiode array 13 is well known in the art. An example of the art of error correcting circuitry is disclosed in the H. E. Murphy US. Pat. No. 3,532,892 entitled Circuit for the Determination of the Centroid of an Illuminated Area and assigned to the assignee of this invention.

Although only four diodes 14,. 15, 16, and 17 are shown in array 13 in the example for the purposes of locating a drifting electron beam, obviously many more diodes could be employed. The number of four was merely chosen for illustration. It is particularly helpful to have more diodes located at the diagonals of the array to be sure to detect a drifting beam irrespective of the angle to which it happens to drift. The connections of the additional diodes are exactly the same as for the illustrated four diodes; each error diode (like diodes l4, 15, 16, and 17), when generating an output signal, triggers the proper beam deflection and correction circuit to move the beam back onto the trace.

The diode array 13 can also function as an automatic focus and astigmatism corrector. A beam focused for the smallest roundest spot will not strike any off center diodes. However, if the spot becomes defocused, the beam diameter would expand and strike peripheral diodes in all directions. If the beam becomes astigmatic, it will strike diametrically opposite diodes only. A drift in focus or astigmatism will be detected and corrected by an error signal through a servomechanism to the beam focusing and astigmatism circuitry (which is well known in the art). Where dynamic focus and astigmatism correction is used, the feedback voltages will adjust a shift in the dynamic correction curves. Simultaneous position, focus and astigmatism correction is maintained by the servo feedback and correction circuitry. The particular error is distinguished by the unmistakeable nature of the feedback signals: for positioning errors, single or adjacent diodes will be struck; for focus error, diametrically opposite diodes will be struck in all directions; for astigmatism error, diametrically opposite diodes will be struck in opposite directions.

Although the invention has only been shown with a general embodiment, various improvements and modifications can be made without departing from the essential aspects of the invention. Therefore the only limitations which should be placed upon the scope of the invention are those set forth in the claims which follow.

What is claimed is:

1. Apparatus for detecting and correcting drift of an electron beam in a cathode ray tube display system having conventional beam deflection circuitry, comprising:

a diode array means placed substantially adjacent the in sets of output signals being directly a result of said beam striking or failing to strike one or more diodes which normally would not have been struck, or which normally would have been struck, respectively, by said beam if said beam were on target; and

means coupled to said beam deflection circuitry for correcting the deflection of said beam in response to signals from said diode array indicating said beam has drifted off target, thereby reducing beam drift and improving the beam deflection accuracy of the system.

2. The apparatus of claim 1 further characterized by said cathode ray display system having a portion of the face of the cathode ray tube exposed to viewing and a portion thereof blocked from view, the diode array means being placed in said portion which is blocked from view.

3. The apparatus of claim 1 further characterized by said diode array means being placed such that said electron beam fails to strike any of said diodes when said beam is on target, but strikes one or more of said diodes when said beam has drifted off target.

4. The apparatus of claim 3 further characterized by the addition of at least one more diode, said additional diode providing a positive output signal when said beam is on target and failing to provide such a signal when said beam has drifted off target.

5. The apparatus of claim 3 further characterized by said diode array means providing focus and astigmatism correction signals. 

1. Apparatus for detecting and correcting drift of an electron beam in a cathode ray tube display system having conventional beam deflection circuitry, comprising: a diode array means placed substantially adjacent the useable face area of a cathode ray tube in an area scanned by the electron beam such that said diode array produces directly a first set of predetermined output signals when said beam is properly directed and is substantially free from drift, but produces any one of a plurality of different sets of output signals if said beam has drifted off target, the change in sets of output signals being directly a result of said beam striking or failing to strike one or more diodes which normally would not have been struck, or which normally would have been struck, respectively, by said beam if said beam were on target; and means coupled to said beam deflection circuitry for correcting the deflection of said beam in response to signals from said diode array indicating said beam has drifted off target, thereby reducing beam drift and improving the beam deflection accuracy of the system.
 2. The apparatus of claim 1 further characterized by said cathode ray display system having a portion of the face of the cathode ray tube exposed to viewing and a portion thereof blocked from view, the diode array means being placed in said portion which is blocked from view.
 3. The apparatus of claim 1 further characterized by said diode array means being placed such that said electron beam fails to strike any of said diodes when said beam is on target, but strikes one or more of said diodes when said beam has drifted off target.
 4. The apparatus of claim 3 further characterized by the addition of at least one more diode, said additional diode providing a positive output signal when said beam is on target and failing to provide such a signal when said beam has drifted off target.
 5. The apparatus of claim 3 further characterized by said diode array means providing focus and astigmatism correction signals. 